This relates generally to image sensors, and more specifically, to the storage capacitance of photodiodes within image sensors.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
Image sensors often include a photodiode having a pinning-voltage which is a design parameter set by the doping levels of the photodiode. During normal operation, a photodiode node is first reset to the pinning-voltage using transistor circuitry. Then photons are allowed to enter the photodiode region for a pre-defined amount of time. The photons are converted to electrons inside the photodiode volume, and these electrons reduce the reset pinning-voltage. In this process, the maximum total charge stored, QMAX, is commonly referred to as the saturation full well (SFW) and depends on the well capacity of the photodiode. The actual charge stored, Q, is less than or equal to QMAX based on the intensity and integration time of photons. When it is time to read out the stored signal, the stored charge Q at the photodiode node is transferred to a floating diffusion node through additional transistor circuitry. Pixel design should maximize the amount of charge Q that can be transferred from the photodiode to the floating diffusion node. If not, the charge spill back manifests as a loss to image quality. Maximum charge stored, QMAX, determines the highest signal level detected in the photodiode array. High QMAX improves the dynamic range of an image sensor.
There are many sources of noise that may degrade the captured signal Q. Dark-current refers to electrons generated and captured by a photodiode from non-photon sources. Dark-current can originate from many sources including: Si defects due to implant & plasma damage, metallic contaminants in photodiode volume, avalanche and/or Zener high field electron-hole (e-h) pair generation, SRH e-h pair generation, trap related band-to-band-tunneling (BTBT), transfer gate induced BTBT on both photodiode and floating diffusion sides, and many others. In order to achieve high image quality, dark-current must be reduced. Lower dark-current improves signal to noise ratio (SNR) of the image sensor.
It would therefore be desirable to be able to achieve very high photodiode well capacity and very low dark current without sacrificing image quality.